Check valves per se are well known in the art. Such valves function as a one-way gate in a fluid line by permitting fluid to flow through the valve in one direction while checking fluid flow through the valve in the reverse direction. Check valves are commonly employed in fluid systems where one needs to be certain that the fluid in the system will flow through the pipes in only one direction, e.g. a domestic fresh-water line, or a waste disposal line.
Many designs now exist for check valves. One of the most common designs comprises a cylindrical valve casing which has a fluid passageway extending along its major axis, a valve seat disposed about the perimeter of one segment of the passageway, a valve member movably disposed within the passageway, and spring means for yieldably urging the valve member into engagement with the valve seat so as to close off the passageway. This construction acts so that when fluid flows into the passageway in a first direction whereby the pressure of the fluid acts to reinforce the spring bias on the valve member, the valve member will remain firmly seated on the valve seat and the passageway will be closed off to fluid flow. At the same time, however, when fluid flows into the passageway in a second opposite direction whereby the pressure of the fluid works against the spring bias on the valve member, the valve member can disengage from the valve seat and open up the passageway to fluid flow so long as the pressure exerted on the valve member by the fluid flow is greater than the pressure exerted on the valve member by the spring means. As a result, when a suitable check valve of this construction is inserted into a fluid line, the valve will act as a one-way gate in the line and permit fluid flow through the valve in one direction while checking fluid flow through the valve in a reverse direction.
Check valves utilizing such a design have rendered excellent service in many piping systems. Unfortunately, however, they suffer from one serious disadvantage. When such a check valve has been installed in a fluid line and the flow of fluid through the valve ceases, an observer monitoring the "downstream" side of the valve cannot be certain precisely why the flow has stopped: the flow through the valve could have stopped because there is insufficient fluid pressure present on the "upstream" side of the valve to move the valve member off the valve seat, or the flow could have stopped because debris and/or mechanical malfunction of the valve impedes the disengagement of the valve member from the valve seat despite the presence of the requisite fluid pressure on the "upstream" side of the check valve. This distinction as to why the valve is not passing fluid is important, since in the former case the valve is functioning properly and will require no servicing, whereas in the latter case the valve is not operating normally and will require servicing or replacement. As a result, it has sometimes been necessary to disconnect the fluid line on the "upstream" side of the check valve to determine if there is normal fluid pressure upstream of the check valve. Alternatively a sampling valve may be installed upstream of the check valve, so that an observer can monitor any fluid entering the check valve and, in combination with observations made on the "downstream" side of the valve, determine precisely why no fluid is passing through the check valve. Then the observer will be able to have the check valve serviced or replaced if need be.
In practice, the sampling valve can be of fairly simple construction. Nonetheless, having any sampling valve at all in addition to a check valve is undesirable because of added cost and because it introduces another element into the system which could fail and thereby create complications. The cost factor has discouraged use of sampling valves in domestic water supplies.